Hexagonal spin structure of A-phase in MnSi: Densely packed skyrmion quasiparticles or two-dimensionally modulated spin superlattice?

We have studied in detail the A-phase region in the field-temperature (H-T) phase diagram of the cubic heli-magnet MnSi using small angle neutron diffraction. The A-phase revealed itself as a two-dimensional hexagonal pattern of Bragg spots with k _{h(1, 2, 3)} ⊥ H. The directions and magnitudes of the wave vectors k _{h(1, 2, 3)} are well preserved over the whole crystal of the size of 100 mm³, but in the small room of the (H-T) phase diagram just below T _c = 29 K. The droplets of the orientationally disordered, presumably hexagonal, spin structure with k _h ⊥ H are observed in the wide range beyond the A-phase boundaries in the field range from B _T1 ≈ 0.1 T to B _T2 ≈ 0.25 T at temperatures down to 15 K. No melting of these droplets into individual randomly located skyrmions is observed for all temperatures and magnetic fields. The wave vector of two-dimensional modulations k _h is equal to the wave vector of the cone phase k _c . We conclude that observable is a two dimensionally modulated hexagonal spin superlattice built on the same competition of interactions (ferromagnetic exchange and Dzyaloshinskii-Moriya interactions) similar to a case of one-dimensionally modulated simple spin spiral.     

Microwave absorption in high-T c superconductors at high magnetic fields

In the mixed state of superconductors (H _c1?H?H _c2) the penetration of microwaves is governed by both, complex conductivity σ(T) and driven oscillation of vortices. In this paper, we show that an effective microwave conductivity can be derived and used to fit the field dependences of the surface resistanceR _s. The fit parameter is the upper critical field. Measurements on single crystals YBa₂Cu₃O_7-δ were made in magnetic fields (H∥c) up to 2.25 T, and in the temperature range from 70–100 K. The critical temperature for mean field superconductivity appears to be 89.3 K, while the apparent onset in the curve ofR _s(T) appears at about 92 K. The magnetic dependences ofR _s clearly demonstrate that one can separate the regions of mean field superconductivity from the region of fluctuations.     

Conversion coefficients and E0 transitions in102, 104, 106Pd

In in-beam (p, p′) experiments, electron and γ-spectra were measured in the electron energy range of 500-1840 keV for¹⁰²Pd and¹⁰⁴Pd, and 600–1580 keV for¹⁰⁶Pd. The conversion coefficients of all transitions in this range were obtained with accuracies of about 20%, in some favourable cases 10%. Special attention was given to 0⁺′-0⁺ transitions from the two-phonon triplets to the ground states with the following results for the branching ratios 0⁺′-0⁺ (ground state) to 0⁺′-2⁺ (one-phonon state):¹⁰²Pd:T _k (E0)/T _γ (E2)<(2.1±3.6)·10^{?7 104}Pd:T _k (E0)/T _γ (E2)=(6.0±1.4)·10^{?5 106}Pd:T _k (E0)/T _γ (E2)=(6.0±2.0)·10^?4     

Re-entrant superconductivity in a magnetically ordered superconductor: La1-xGdxRu2

We have measured the critical temperature (T_c) and the upper critical magnetic field (H_c2) of La_1-xGdxRu₂. At low concentrations of the magnetic impurity (Gd), the suppression of T_c follows the expected Abrikosov-Gorkov (A-G) pair breaking curve. However, for larger concentrations, strong deviations below A-G are observed. Samples in this region (4. ? × ? 5. at. %) exhibit two T_c's. La_1-xGd_xRu₂ is known to order magnetically, probably as a spin glass, and the magnetic ordering temperature (T_M) has been measured in the normal state. This T_M curve intersects the T_c curve in the concentration range where the T_c curve is re-entrant and we therefore attribute the re-entrant T_c behavior to the magnetic ordering of the Gd³⁺ ions.     

Calculation of the surface parallel critical field for clean superconducting films

On the basis ofSchöler's theory of boundary conditions for the pair potential in clean superconductors, corrections are obtained to standard results forH _c∥. They are given explicitely both for thin and thick films. The results should be valid forT in the neighbourhood ofT _c . The thin film corrections do not vanish even in the limitT→T _c .     

Anisotrophy of X-ray critical scattering in liquid NPOB crystal

We have carried out a high-resolution X-ray critical scattering experiment in the nematic phase connected with the nematic → smectic A transition in 4-nitrophenyl-4-n-octyloxybenzoate (C₈H₁₇O-C₆H₄-COO-C₆H₄-NO₂)-NPOB. The measurements yielded the following parameter values: $\text{d =}\text{30.20}\text{A}\text{?}$, $\text{q}{}_{\mathrm{o}}\text{=}\text{0.205}\text{A}\text{?}{}^{\mathrm{?1}}$ and the critical expons γ = 1.31 ± 0.11, γ_∥ = 0.69 ± 0.06, γ_⊥ = 0.49 ± 0.08, for 1.5 x 10^?4 ? (1?T/T_c) ?10^?2. the ratio of the longitudinal to the transverse correlation lengths ξ6/ξ⊥ increases gradually with decreasing reduced temperature and it changes by a factor of about two within the above temperature range.     

Local magnetization study of the first-order superconducting transition in CeCoIn5

We report local magnetization measurements on the heavy fermion superconductor CeCoIn₅ using a micro-Hall probe. Below a critical point T₀ (), the local magnetization shows a clear jump at the superconducting transition temperatures for both H∥a and H∥c, indicating that the phase transition at the upper critical field H_c2 becomes a first-order phase transition. In addition, we observed an undershoot behavior of magnetization jump above (H∥c), which suggests a rapid change of textured superconducting structure with vortices and the nodal planes expected in the Fulde-Ferrell-Larkin-Ovchinnikov state.     

Upper critical fields of superconducting Gd and Tm doped LaSn3: Effects of crystalline electric fields

The temperature dependence of the upper critical fields, H_c²(T), are presented for (La_1-xGd_x)Sn₃ and (La_1-xTm_x)Sn₃. For samples with nearly the same T_c, H_c²(T) of the Tm-doped LaSn₃ samples are always larger than those for the Gd-doped samples. The results are interpreted in terms of crystalline electric field splitting of magnetic levels of the Tm³⁺. Pure LaSn₃ is found to be a Type I superconductor.     

Intrinsic low field susceptibility in pseudobinary DyxY1−&gt;x&gt;Al2andTbxGd1−xAl2 cubic intermetallic compounds

Low field (H_ac ≈ 3.5 x 10^-6T) and low frequency (15 Hz) ac susceptibilities, χ_i, have been measured between 3.3 K and well above the Curie temperatures, for the intermetallic Laves phase compounds Dy_xY_1-xAl₂ and Tb_xGd_1-xAl₂. It is concluded that χ_i is, at low enough temperatures, dominated by the domain wall excursions around the minima of the anisotropy Peierls potential wells. The derived model relation χ_i(T)H_c⁽ⁱ⁾(T) = const., where H_c⁽ⁱ⁾ is the intrin sic pinning coercive field, is confirmed at low enough temperatures. The shape of the observed susceptibility cusps around the Curie temperatures in the series Dy_xY_1-xAl₂ (x 0.35) suggests that these systems are magnetically inhomogenous. The magnetic domain sizes estimated from these measurements seem quite small (between around 0.01 and 0.1 μm).     

Spin relaxation parameters in recombining radical ion pair (diphenylsulfide-d10)+/(p-Terphenyl-d14)− obtained by OD ESR and quantum beats techniques

Parameters of paramagnetic relaxation were determined by OD ESR and quantum beats techniques for a recombining pair of radical ions (DPS-d₁₀)⁺/(PTP-d₁₄)^? inn-hexane, isooctane,cis-decalin, and squalane solutions. TheT ₂ relaxation time determined by quantum beats technique is independent of solvent viscosity and magnetic field strength in the range 170–9600 G. These data are in agreement with the results obtained by OD ESR technique assuming fastT ₁ relaxation for radical cation. Neglecting the contribution of radical anion relaxation, we obtainedT _1c=T _2c?50 ns for (DPS-d₁₀)⁺.     

Gleichzeitige Messung von Hyperfeinstruktur,Starkeffekt und Zeemaneffekt des85Rb19F mit einer Molekülstrahl-Resonanzapparatur

A molecular beam resonance apparatus with electric quadrupole lenses asA- andB-fields and with superimposed parallel electric and magnetic transition-fields was used. Molecules in different rotational statesJ, m _J are separated by theA-field. Spectra of molecules in different vibrational states are resolved by their different Starkeffect energies. By this means the following electric and magnetic properties of the molecule could be measured in the rotational stateJ=1 and vibrational statesv=0 and 1: The magnetic and electric dipole moment of the molecule, the scalar and the tensor nuclear dipole — dipole interactiond _s andd _T, the nuclear spinrotational interactionc _F andc _Rb, the nuclear quadrupole interactioneqQ, the nuclear magnetic moment μ_Rb, the anisotropy of the diamagnetic susceptibility ξ_⊥?ξ_∥, the anisotropy of the diamagnetic shielding of the external field by the electrons at the position of the nuclei σ_⊥?σ_∥. Using these quantities it was possible to calculate the quadrupole moment and a weighted quadrupole moment of the electronic charge distribution. The results are: (J=1,v=0) μ_el=8,5464 (17) debμ _J/J=?29,79(2)x10^?6 μ _B d _s/h=0,36(23) kHzd _T/h=0,69(22)kHzc _F/h=10,42(70) kHzc _Rb/h=0,479 (48) kHz.eqQ _Rb/h=?70,3410(26) MHzμ(1?σ_S)Rb=1,3474(5) μ_k (ξ⊥-ξ _∥)=12(6)×10^?30 erg/Gauß² (σ_⊥-σ∥)_Rb=?3,8(2,1)×10^?4 (σ⊥-σ _∥)_F=?2,6(3)×10^?4     

NMR of 89Y in normal and superconducting YBa2 Cu3 O9-δ

Pulsed NMR data for ⁸⁹Y in the high-T_c superconductor YBa₂Cu₃O_9-δ (δ ≈ 2.1) in both the normal and mixed states are presented. Spin-lattice relaxation exhibits a Korringa temperature dependence in the normal state, with T₁T = (4.6 ± 0.1) × 10³ s-K. Below T_c, T₁ increases much more quickly than for a conventional superconductor. From linewidth measurements, the zero-field penetration depth is estimated to be λ ≈ 3800 Å. The Knight shift in the normal state is small, K ≈ 0.02%. Spin-spin relaxation times, determined by Y-Cu dipolar interactions, decrease below T_c due to the detuning of Cu-Cu interactions by fluxoid field gradients.     

Improved critical current density in Zn doped YBCO single crystals

Magnetic measurements were made using pure YBCO and Zn doped YBa₂(Cu_1?xZn_x)₃O_7?σ. Single crystals with Zn concentration of 0.5%, 1.5%, 3.0% and 4.3%. The magnetic hysteresis loops for these samples were measured in the temperature range 0.1 ? T/T_c ? 0.96 under magnetic fields of 5 T using SQUID. It was found that the critical current density J_c increased for low Zn content samples up to 3% Zn concentration compared to pure YBCO sample and decreased for the higher Zn content samples. These values varied consistently when compared at magnetic fields of 1 T and 3 T. Moreover Zn doped samples showed significant values of J_c in the temperature range of 0.7–0.9T_c, close to critical temperature compared to pure YBCO sample. The irreversibility field H_irr was also enhanced in this temperature range showing consistent decrease with increase of Zn concentration. The peak field H_p above H_c1 and irreversibility field H_irr, both show power law dependence of the form H = m₁(1 ? T/T_c)^m2 in the temperature range of 0.75–0.96T_c. The values of parameter m₂ increased from 1.44 to 1.95 for the samples up to 3% Zn content and decreased to 1.37 for higher Zn contents. The ratio H_irr/H_p was found to be 3–4 for the lower Zn content samples and was 7–8 for the sample with high Zn content indicating more disorder for higher Zn content samples. The region between peak field H_p and irreversibility field H_irr was broadened with the increase of Zn concentration. The strong effect of Zn substitution in modifying behavior of these samples even at elevated temperatures is possibly due to the changes in the anisotropy of our samples with the increase of Zn concentration and also due to the locally induced changes in magnetic moments by Zn substitution.     

Process of magnetic field penetration into the high-temperature superconductor YBa2Cu3O7−δ

The processes of magnetic field penetration into the ceramic samples of the HTSC YB₂Cu₃O_～6.95 at T<T _c are studied by the methods of internal friction and magnetization measurements. A clearly manifested correlation is observed between the field dependences of the internal friction spectrum parameters (the logarithmic damping decrement Q ^?1 and the resonance frequency f) and the trapped magnetic flux ΔM. The magneto-mechanical approach we used reveals a significant difference in the field dependences of the densities of pinned (N _p) and free (N _f) Abrikosov vortices for H>H _c1.     

High-resolution heat capacity of SnCl2·2H2O single crystal

The heat capacity of SnCl₂·2H₂O single crystal was measured in the close vicinity of the phase transition temperature, T_c = 217.994 ± 0.01 K. Its anomalous part ΔC could be expressed as ΔC = A_± ⊥ (T ? T_c)/T_c⊥^-α_±, where α₊ = 0.492 ± 0.02, α_- = 0.492 ± 0.02, A₊ = 1.148 JK^?mol^?, and A_- = 1.155 JK^?mol^?. A quasi-isothermal absorption of the enthalpy amounting to 34 J mol^? was observed at T_c.     

Nuclear magnetic resonance studies of CePd2In and LaPd2In at low temperatures

We report on In nuclear magnetic resonance (NMR) and nuclear quadrupolar resonance (NQR) measurements on the new heavy-electron compound CePdZIn over a wide temperature range, from 45 mK up to 30 K. CePd₂In undergoes an antiferromagnetic (AF) phase transition at T _N = 1.23 K involving small localised Ce moments of 0.11 μ_B. In zero field, the spin-lattice relaxation rate T ₁ ^?1 (T) shows remarkable changes in its temperature dependence. Above 3 K, T ₁ ^?1 is constant and 850 sec^?1. Between T _N and 2T _N, (T ₁ T)^?1 = 330 (Ksec)^?1, but rapidly decreases below T _N. A Korringatype relaxation, characteristic for simple metals at low temperatures, with (T ₂ T)^?1 = 17(Ksec)¹ is resumed below 0.6 K. This value is an order of magnitude larger than (T ₁ T)^?1 for LaPd₂In and therefore is associated with low-energy excitations of the itinerant charge carriers with 4f symmetry. The T ₁ ^?1 data at various non-zero magnetic fields fall on a single curve when plotted as a function of (T/H) if H exceeds 3.5 T. Thus the AF ordering, the 4f moment fluctuations and the Kondo screening are drastically suppressed by the application of fields H of the order of 3.5 T.     

Phase transitions in classical vector models

The equation of state obtained earlier /6/ for 3-dimensional Ising ferromagnets in the first order in$\text{1}\text{z}$ (z is the number of the nearest neighbours) is generalized with regard to the classical D-component vector model. The results obtained are valid in a wide temperature range |τ|?[z²(D+2)]^?1 (τ=(T_c-T)/T_c) and become exact in the limit D → ∞.